This invention relates to an apparatus and a method for applying insulation to power generator stator bars and the resulting stator bars. More particularly, it relates to an apparatus and a method for extruding thermoplastic insulation onto stator bars, an extrusion die and the stator bar comprising thermoplastic insulation that is produced by the method.
Stator bars conduct current out of the generator. Typically, a generator comprises a rotor that rotates in a magnetic field, thereby inducing an electrical field in a conductor. The stator bar is the conductor.
These stator bars typically comprise numerous strands of copper bundled together. The stator bars are manufactured in various lengths, shapes and cross sections, depending on the generator design, voltage and power. The stator bars of each generator are unique, being custom designed for the specific generator. Generators can contain more than 100 stator bars. The stator bars can be up to 30 feet long and several inches in width.
Because of the high voltage carried by these stator bars, they must be insulated electrically from the rest of the stator. Typically, high voltage insulation for generator stator bars is made by a taping process. Multiple layers of any thermosetting epoxy/mica/glass tape are wrapped around the stator bar and then covered with subsequent layers of a sacrificial polymer that protects the insulation layers during later processing. The wrapped stator bars are then heated under vacuum to remove most of the residual solvent from the epoxy resin. The epoxy resin is cured under pressure using conditions that are designed to allow the epoxy to flow sufficiently to fill any voids present in the wrapped layers. In a different process, multiple layers of mica containing tape are wrapped around the stator bar. Then, in a subsequent operation, the bar is vacuum dried to remove air and volatiles followed by pressure impregnation with an epoxy or silicone material. While providing an excellent electrical insulation when properly manufactured, these processes are very time consuming and labor intensive. Also, because of the variable processing parameters, such as time, temperature and pressure, needed to balance the proper amount of solvent release and the degree of epoxy flow prior to full cure of the epoxy resin, these systems are prone to producing an insulation that is incompletely cured or possesses residual voids.
It is desirable to have an insulation that meets the thermal, mechanical and electrical property requirements of the stator bar environment and that can be applied to the stator bar using a method that leaves it essentially free of voids. Therefore, it is desirable to use a thermoplastic material as insulation. It is also desirable to provide a method of applying insulation that is not so labor intensive, and therefore, is cheaper, while at the same time offering thermal and electrical properties that are better than those available with the process of the prior art.
Using extruded thermoplastic as insulation on wiring is well known in the art. Typically, wire is coated by an extrusion head wherein the wire is passed through a central bore within the extrusion head. This central bore is coaxial with one or more annular extrusion mandrels that deposit concentric layers of thermoplastic, such as polyethylene, onto the wire as it passes through the extrusion head. The walls of the central bore are typically parallel to the direction of movement of the wire so that the wire is held substantially straight within the extrusion head. Such an extrusion head cannot accommodate a wire that is not flexible or is not "straight" in the sense that it can be placed into the extrusion head parallel to the direction of the movement of the wire. A wire coating extrusion head is typified by the die disclosed in U.S. Pat. No. 4,940,504 wherein a plurality of layers of plastic compounds are extruded onto a moving electrical conductor which is centered in the plastic coating.
It is desirable to provide an extrusion head that can coat wires or bars which am not flexible enough to be coated through an extrusion head of the prior art. More particularly, it is desirable to provide an extrusion head that can accommodate bars of varying geometric shapes.